Such a conventional data relay method provides service by an IP-VPN, which is service equivalent to service provided by a private line, obtained by establishing a logical tunnel on, for example, a backbone IP network of an ISP. For instance, an intranet such as an intracorporate network, that has been set up conventionally by using a private line, is constructed by utilizing this IP-VPN service of the ISP.
In such IP-VPN service, networks constructed by terminals (host devices) such as PCs of plural users share the backbone IP network of the ISP. Therefore, the IP-VPN service has an advantage such that a network can be constructed at lower cost as compared to the service through a private line singly used by a user. Further, the IP-VPN service is relay service using the Layer 3 (Network layer) level, and also provides a connection service between LANs that relays through LANs of users at the Layer 2 (Data Link layer) level to make a connection between the LANS.
An ADSL (Asymmetric Digital Subscriber Line) being widespread based on an Internet access network or an optical fiber access network uses technology called PPPoE (Point to Point Protocol over Ethernet), and host devices of users and an ISP are connected through a network set-up by an access-network service enterprise (hereafter called “access network”). At this time, tunneling technology called L2TP (Layer 2 Tunneling Protocol) or MPLS (Multiprotocol Label Switching) is used to provide one-to-one communication within the access network. In this tunneling technology, a new header is added to data communicated between the host device of a user and the ISP within the access network to encapsulate the data, and the data is relayed within this access network.
FIG. 20 is a schematic constructional diagram showing a system construction of a conventional Internet access network that relays data via the ADSL. In this figure, the access network 20 is set up by the internally provided network terminating device 21 and BAS (Broadband Access Server) 22, and the logical tunnel 23 is established between the network terminating device 21 and BAS 22. The IP network 10 of the ISP is connected with the network terminating device 21 through a line L, and the host devices 27 to 29 of users A to C are connected to the BAS 22 through respective ADSLs (telephone lines) 24 to 26. The data transmitted from the IP network 10 of the ISP is input into the network terminating device 21 through the line L, where the data is subjected to relay processing, and the data is forwarded to the BAS 22 through the logical tunnel 23, and distributed to the respective host devices 27 to 29.
That is, when data is to be distributed from the IP network 10 of the ISP to each user, the data transmitted from the IP network 10 of the ISP is loaded into the network terminating device 21 in a packet structure consisting of IP header and data. This packet is added with header for tunnel within the access network 20 (IP header within an enterprise network) and PPP (Point-to-Point Protocol) header in the network terminating device 21, and is transmitted to the BAS 22. The BAS 22 removes only the header for tunnel from the packet and transmits the packet as PPP data to the respective host devices 27 to 29 through the ADSLs 24 to 26.